Physical science is the _knowledge_ of the world by their means. It is a rival, not of them, but of the medley of prior dogmas, superstitions, and chance opinions about the world--a medley which grew up and nourished precisely because of absence of a will to explore and of a technique for detecting unambiguous data. That Mr. Russell, who is a professed realist, can do no better with the problem (once committed to the notion that sense objects are of themselves _objects_ of knowledge) than to hold that although the world of physics is not a legitimate inference from sense data, it is a permissible logical construction from them--permissible in that it involves no logical inconsistencies--suggests that the pragmatic difference between idealist and realist--of this type--is not very great. From necessary ideal constructions to permissible logical constructions involves considerable difference in technique but no perceptible practical difference. And the point of this family likeness is that both views spring from regarding sense perception and science as ways of knowing the same objects, and hence as rivals until some scheme of conciliation has been devised. _c_) It is but a variant of this problem to pass to what may be called either the ego-centric predicament or the private-public problem. Sense data differ from individual to individual. If they are recognized to be natural events, this variation is no more significant than any change depending upon variation of generating conditions. One does not expect two lumps of wax at different distances from a hot body to be affected exactly alike; the upsetting thing would be if they were. Neither does one expect cast-iron to react exactly as does steel. That organisms, because of different positions or different internal structures, should introduce differences in the phenomena which they respectively have a share in producing is a fact of the same nature. But make the sense qualities thus produced not natural events (which may then be made either objects of inquiry or means of inquiry into something else) but modes of knowing, and every such deviation marks a departure from true knowing: it constitutes an anomaly. Taken _en masse_ the deviations are so marked as to lead to the conclusion (even on the part of a realist like Mr. Russell) that they constitute a world of private existences, which, however, may be correlated without logical inconsistency with other such worlds. Not all realists are Leibnizian monadists as is Mr. Russell; I do not wish to leave the impression that all come to just this solution. But all who regard sense data as apprehensions have on their hands in some form the problem of the seemingly distorting action exercised by the individual knower upon a public or common thing known or believed in. IV